MnO₂ Nanorods on Mesoporous Carbon as a Bifunctional Electrocatalyst for Hydrazine Oxidation and Oxygen Reduction Reactions in Alkaline Media

Abstract

Hydrogen production via water splitting is instrumental in harvesting zero-carbon green energy. Hydrazine oxidation reaction (HzOR) is recognized as a replacement for the slothful anodic oxygen evolution reaction. However, agonizingly, HzOR gets hampered due to its high working potential in contrast to its low theoretical value and the usage of high-cost precious metal-based electrocatalysts to lower the overpotential. Additionally, the slow nature of the cathodic oxygen reduction reaction (ORR) is experienced by noble metal (Pt)-based catalysts, which have the shortcomings of being uneconomical and suffer from the methanol crossover effect. Thus, developing nonprecious metal-based economic bifunctional electrocatalysts catalyzing HzOR and ORR is imperative. Herein, we designed a simple strategy to synthesize manganese oxide nanorods loaded in mesoporous carbon (CKT) as a bifunctional electrocatalyst for boosting the anodic HzOR and cathodic ORR in alkaline media. The as-synthesized MnO 2 /CKT nanocomposite shows excellent response toward HzOR with a low onset potential of 0.7 V vs RHE and achieved a current density of 10 mA cm −2 at a potential of 0.88 V vs RHE. MnO 2 /CKT nanocomposite also displayed a Tafel slope (105 mV decade −1 ) and a maximum current density of 25.6 mA cm −2 at a potential of 1.4 V vs RHE in the presence of 50 mM hydrazine hydrate in 1 M KOH. Furthermore, the MnO 2 /CKT electrocatalyst can also reduce molecular oxygen with similar efficiency to the state-of-the-art Pt/C catalyst through an efficient 4-electron pathway with an improved onset potential of 0.95 V vs RHE and a current density of −4.5 mA cm −2 in O 2saturated 0.1 M KOH. The electrochemical impedance spectroscopy and chronoamperometric (i−t) responses of the electrocatalyst showed the feasibility of rapid electron transfer and high stability at the interface. The promising bifunctional electrocatalytic efficiency of the unique MnO 2 /CKT nanocomposite initiates from the synergetic physicochemical characteristics of MnO 2 and CKT, enabling smooth analyte diffusion with low charge transfer resistance and offering a superior quantity of active sites for the catalytic reactions.